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Overlapped Electromagnetic Coilgun for Low Speed Projectiles

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Overlapped Electromagnetic Coilgun for Low Speed Projectiles ISSN (Print) 1226-1750 ISSN (Online) 2233-6656 Journal of Magnetics 20(3), 322-329 (2015) http://dx.doi.org/10.4283/JMAG.2015.20.3.322 Overlapped Electromagnetic Coilgun for Low Speed Projectiles Hany M. Mohamed1, Mahmoud A. Abdalla2*, Abdelazez Mitkees, and Waheed Sabery Electrical Engineering Branch, MTC College, Cairo, Egypt [email protected] [email protected] (Received 20 February 2015, Received in final form 16 June 2015, Accepted 16 June 2015) This paper presents a new overlapped coilgun configuration to launch medium weight projectiles. The proposed configuration consists of a two-stage coilgun with overlapped coil covers with spacing between them. The theoretical operation of a multi-stage coilgun is introduced, and a transient simulation was conducted for projectile motion through the launcher by using a commercial transient finite element software, ANSOFT MAXWELL. The excitation circuit design for each coilgun is reported, and the results indicate that the overlapped configuration increased the exit velocity relative to a non-overlapped configuration. Different configurations in terms of the optimum length and switching time were attempted for the proposed structure, and all of these cases exhibited an increase in the exit velocity. The exit velocity tends to increase by 27.2% relative to that of a non-overlapped coilgun of the same length. Keywords : electromagnetic launch, excitation circuit, lorentz force, overlapped coilgun 1. Introduction is not easy to obtain the main performance parameters and optimize the design [3]. Electromagnetic (EM) launch technology is a strong Sandia National Laboratories has succeeded in coilgun candidate to launch objects with high velocities over long design and operations by developing four guns with distances. A coilgun launcher is a specific application for projectiles ranging from 10 g to 5 kg and speeds of up to an electromagnetic launch that uses a moderate power 1 km/s, validating the computational codes and basis for excitation circuit [1]. A coilgun consists of discrete gun system control. In addition, coilguns use magnetic solenoidal coils that are connected end-to-end and are coupling to drive the current in the armature without wound around a hollow dielectric tube. A projectile inside requiring direct electrical contact between the barrel and this tube moves due to the presence of coupling between the projectile. This results in a long barrel lifetime that the coils and the armature. Capacitor bank circuits are will be useful for next-generation long-range artillery connected to the coils that are triggered sequentially to guns for land-based and naval platforms [4]. deliver current pulses into the coil's stages, and as the A coilgun launcher can be designed as a multi-stage current pulse travels through the coil’s winding, the structure. This kind of launcher can be referred to as a interaction between the magnetic field and the induced synchronous induction coilgun (SICG) or also a coaxial current within the armature (projectile) generates an axial coil launcher. It consists of a sequence of powered multi- force that moves the projectile along the barrel. A number turn drive coils that surround a barrel with a circular cross of parameters affect the performance and efficiency of the section through which a conducting armature passes [5]. coilgun including: the muzzle velocity, projectile peak The drive coils must be separately energized by the and average acceleration [2]. Although the working external source, which is controlled by switches that are principle of a coil gun is simple, the launch of a coil gun triggered to sequentially deliver current pulses into the is a complex electromagnetic transient process, and so it coil stages. The time to turn the switches on and off must be governed by the position of the armature [6]. This type of launcher has several advantages that result ©The Korean Magnetics Society. All rights reserved. in a higher acceleration, including no drop in acceleration *Corresponding author: Tel: +2-01118750114 Fax: +2-01118750114, e-mail: [email protected] as the mass of the projectile increases, a higher peak © 2015 Journal of Magnetics Journal of Magnetics, Vol. 20, No. 3, September 2015 − 323 − pressure on the projectile and smaller differences between the average and peak pressure. The analytical approach for an inductive coilgun is relatively complex due to the time-varying mutual inductances between the driver coils and the projectile in the transient launch process. Hence, it is necessary to rely on finite element software or a numerical approach to design a coilgun that can achieve good launch characteristics [7]. In this paper we introduce a new configuration for a multi-stage coilgun that is based on an overlap between Fig. 1. (Color online) Forces that exist on the stator and arma- two cascaded coilgun stages. This configuration is pro- ture coaxial coils for a single stage coilgun. posed in order to achieve a higher launch velocity for a projectile with the same coilgun length. The theoretical projectile, (Jaφ) is the radial armature current density and concepts of the proposed overlapped structure are intro- (Bsr) is the stator radial magnetic field. Hence, the speed duced, and the performance of the new configuration is inside of the projectile resulting from this force can be examined and compared to that of a conventional (non- calculated as [6] overlapped) coilgun. An extensive study was carried out dv dM m --------a I I -----------as- to validate the results, and the simulation of the projectile a dt = a s dz (2) movement through the coilgun is carried out by using the finite element method. dz v ----- a = dt (3) 2. Theory where (ma) is the mass of the armature, (va) is the velocity of the armature, and (Ia, Is) are the induced armature and 2.1. Basic Coilgun Concept stator coil currents, respectively. (z) is the displacement of The electromagnetic coil launcher structure is shown in the armature and (Mas) is the mutual inductance between Fig. 1, and as is shown in the figure, it consists of stator the armature and the stator coil [6]. coil and an armature that moves along the stator coil axis. As shown in the figure, the armature is the projectile 2.2. Multi stage Coil Gun itself, and the stator current density (Js) circulates around In the case of the multi stage coilgun, the analysis can the axis. This current induces a magnetic field inside the be carried using the Current Filament Method (CFM), as armature (Ba), and accordingly, a current density (Ja) is shown in Fig. 2. The projectile and conductive parts of induced in the armature that results in turn in a magnetic the coil are divided into elementary elements (m and n) field around the stator (Bs). The force that affects the within which the projectile can be assumed to be a point projectile inside of the electromagnetic coil is shown in projectile. Hence, the current distribution is uniform. Fig. 1, and the driving force (F) in the coil launcher is the Next, a numerical field computation is used to calculate Lorentz Force [8, 9]. The force (F) can be decomposed the value of the forces that are applied on the projectile into a propulsive (axial) force (Fz) and a radial force (Fr) by determining the flux distribution, magnetic field and that exist on both the armature and the stator. velocity that act on the projectile anywhere inside of the The radial forces of the coilgun that are induced on the system for each small section. Finally, the current filament both stator and the armature are in opposite directions. associated with each volume element and its correspond- These opposing radial forces center the armature inside ing electrical parameters are calculated. the stator coil, making a contactless launch possible. The velocity of the projectile can be calculated using Hence, the projectile is centered and no friction occurs Eq. (3), where the force exerted on the projectile can be between the projectile and the stator walls, and thus, the calculated as [10] radial forces are balanced. Accordingly, the motion of the m projectile inside of the stator is mainly a result of the dva dMasi m I I a-------- = ∑ ai si------------- (4) armature axial force that affects the projectile. This force dt i = 1 dz can be mathematically expressed as where Faz = Jaϕ × –Bsr (1) th Iai : Armature current for the i element point where (Faz) is the armature axial force that pushes the projectile. − 324 − Overlapped Electromagnetic Coilgun for Low Speed Projectiles − Hany M. Mohamed et al. Fig. 2. (Color online) Schematic diagram of the CFM of a coilgun. Fig. 3. (Color online) Coilgun layout (a) two-stage coilgun. (b) two-stage overlapped coilgun. th Isi : Stator current for the i element point projectile. th Zpi : position along the z axis of the i element point projectile. muzzle point. Therefore, we have to select the best position th Masi : Mutual inductance between the i element point to launch the projectile in order to obtain the highest projectile and the coil (stator). muzzle velocity. 2. Studying the effect of the flux linkage between the The knowledge of the magnetic field at an arbitrary coil and armature to ensure the best muzzle velocity of place (Zpi) allows for a simplified form of the mutual the projectile. inductance to be be written as 3. An inversely proportional relationship is observed N2µA Z ()Z l between the length of the coil and the muzzle velocity of M pi pi – asi = ------------- ---------------------- – ---------------------------------- (5) the projectile, so the coil length needs to be optimized to 2l a2 Z a2 ()lZ 2 + pi + – pi obtain a better muzzle velocity. From the velocity definition for the finite length projectile, 4. The effect of the number of turns of the coil, applied a numerical code should be used to solve the projectile voltage and current pulse must be investigated and velocity, flux linkage and mutual inductances.
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